Vista is the gaming platform of choice using crysis and other titles that soon comes out.
there is the ati XT the card to have.

Crossfire solutions will also be one where 8 cards might run at the same time.
Where performance will scale well. Nvidia has nothing there as of now.

Dx9 isnt the design choice for 2900XT as I seen it, all stuff indicate and points to DX10 and up and Vista.
So not suprised if games show a lower number in the begining, drivers will enhance that pretty much even in dx9.
3Dmark indicate the cards works well, so its all drivers to get.